The present invention relates to a small-sized trimmer capacitor, and more particularly to the electrode structure thereof.
In general, a trimmer capacitor is constructed of an insulating substrate, a ground stator plate lying over one half of the upper surface of an insulating substrate, a hot stator plate lying over the other half of the upper surface of the insulating substrate and having a dielectric film disposed between it and the substrate, and a substantially semicircular rotor plate which is adapted to slide over both the stator plates while remaining in electrical contact with the ground stator plate.
The dielectric film is normally adhered to the hot stator plate, and various sythetic resins are typically used. A thermocompression lamination process is normally the preferred method of adhering the dielectric film to the substrate but, unfortunately, there are very few materials which are both excellent in the radio-frequency electrical characteristics and which can also be readily adhered to metals. At the present time a polyethylene film subjected to radiation irradiation or the like is the only practical material.
Although such polytheylene film is very good in the radio frequency characteristics, it is comparatively small in its specific inductive capacity .epsilon..sub.r. In the case where it is intended to make the capacitance value of the trimmer capacitor quite large per unit area, it is necessary to make the thickness of the polyethylene film small and the electrode spaceing short. In this regard, since polyethylene is low in mechanical strength and thermal resistance, the maintenance of an accurate thickness is difficult in the production of the film for thicknesses of and below 25 .mu.m. In consequence, the hot stator plate laminated with the polyethylene film is prone to have a non-uniform thickness and to cause a step or level difference between its surface and the surface of the ground stator plate. This leads to the disadvantage that the close contact and the electric conduction between the ground stator plate and the rotor plate become unstable.